(1) FIELD OF THE INVENTION
The present invention relates to novel polyimides and their polymerizable prepolymerization products as well as to a process of making such polymerizable prepolymerization products and to the use of such prepolymerization products for producing highly cross-linked, insoluble, and infusible polyimide resins of excellent thermal, electrical, and mechanical properties.
(2) DESCRIPTION OF THE PRIOR ART
French Pat. No. 1,555,564 describes the preparation of polyimide prepolymerization products by reacting bis-maleimides of the general formula ##STR1## with difunctional amines of the general formula EQU H.sub.2 N--R.sub.2 --NH.sub.2
Specific and predetermined proportions of said reactants are used in producing the prepolymers. The resulting prepolymers can be converted into insoluble and infusible, crosslinked polyimide resins by heating the prepolymers to a temperature between about 100.degree. C. and about 400.degree. C. until polymerization is completed. When proceeding in this manner, cross-linked polyimides with valuable properties such as remarkable stability at high temperatures, high mechanical strength properties, and excellent dielectric properties are obtained. In said reaction of the bis-maleimides with the difunctional amines, the free amino groups react with the double bond of the maleimide molecule. Prepolymers with terminal maleimide groups are produced by said reaction. Cross-linked polymers are obtained from said prepolymers by the action of heat causing polymerization of the maleic acid double bond. Prepolymerization can be effected in solution or in the melt. The prepolymerization reaction proceeds only at temperatures of about 150.degree. C. at such a speed that a technically satisfactory manufacture is possible. The preparation of uniform prepolymerization products by means of the above described process is rather difficult and can not always be carried out with reproducible results because, during the preparation of the prepolymer at the temperature of 150.degree. C., polymerization of excess maleimide groups takes place to an appreciable extent at the same time.
Furthermore, when proceeding in the manner described in said patent the possibility exists that unreacted difunctional amine is still present. Its presence in the prepolymer, however, is physiologically not without danger on subsequently using the prepolymer.